PROJECT SUMMARY The primary cilium is a microtubule-based signaling organelle with essential roles in many vertebrate tissues. Disruptions in cilium structure or function lead to a broad range of developmental disorders, including some cancers, revealing the critical function of cilia. Cilia are dynamic structures, undergoing assembly and disassembly during each cycle. Although cilium-based signaling can directly regulate the cell cycle, cilia function has been poorly studied in proliferating cells. Therefore, it is not understood how a transient organelle can regulate a sustained process such as proliferation. The goal of this proposal is to address this question in the context of Hedgehog (Hh) signaling, the classic example of a developmental pathway that requires the cilium for signal transduction. I propose to investigate the mechanisms of Hh-dependent proliferation in medulloblastoma (MB), the most frequent childhood brain cancer. MB cells require both cilia and Hh signaling for proliferation, and my preliminary work in an established cell culture model of MB provides evidence of a cilium cycle. Given that MB cells proliferate even though cilia are transient, my central hypothesis is that uncharacterized downstream mechanisms are necessary to buffer Hh-dependent proliferation against changing cilia. I will test this hypothesis with the following aims: 1) I will establish the relationship between Hh signaling and the cilium cycle. Using fluorescent live imaging and Hh signaling assays, I will determine how cilia and Hh signaling change throughout the cell cycle. 2) I will determine when signaling at cilia is necessary and sufficient for cell cycle progression, using live imaging of the cell cycle and small molecule inhibitors of Hh signaling. 3) I will use candidates from a large-scale genetic screen to identify new regulators of Hh-dependent proliferation that constitute novel components of the link between Hh signaling and proliferation as well as modulators of the Hh signaling pathway specific to the physiology of MB cells. The results will reveal how Hh signaling, cilia, and the cell cycle cooperate to drive MB growth, improving our understanding of cilium-based signaling in both normal and disease states.